'79 Seville fuel pump issues

[J.C.]

Hello Everyone,

I'm hearing an irritating electronic humming sound from the rear left driver's-side of my '79 Seville.  The source of the noise is the chassis-mounted fuel pump. 

My mechanic removed that pump and replaced it with a new one but the noise, though reduced, is still there.  So, my mechanic tried lengthening hoses connecting to the pump and repositioning the pump, but the noise is still there.

My mechanic told me a few things that sound a bit odd, though... 

First, he said that he's not able to determine whether there's another fuel pump inside the fuel tank.  But looking at my 1979 Cadillac Service Information manual seems to make clear that there is a fuel pump inside the fuel tank, as seems apparent is the copies of the attached pages.

Second, he said that he doubted there's an in-tank fuel pump because, when his guys disconnected the chassis-mounted fuel pump, the car shut down.

Third, he said that determining whether the car has an in-tank fuel pump would require him to remove the fuel tank and that such a project was large and involved and isn't worth doing because the noise isn't that bad.

Questions:

1.  Isn't it reasonable to suppose that the chassis-mounted fuel pump shouldn't be making that much noise, or is this just how the car is supposed to sound?

2.  Isn't it reasonable to guess that the chassis-mounted fuel pump is making so much noise because it's overworked because it doesn't have the support of the dysfunctional in-tank fuel pump?  And that the car shut-down when the chassis-mounted fuel pump was disconnected because the in-tank fuel pump was broken and unable to assist?

3.  Is it really such a big deal to take the fuel tank out?

4.  Is there any way to determine whether the in-tank fuel pump is working other than by removing the fuel tank?

I'm not a mechanic, so any input would be greatly appreciated!

J.C.

[TC]

There is a primary pump in the fuel tank that acts as a primer or "pusher" to the main fuel pump mounted on the chassis. This is a similar design that many German vehicles used in the 80s. Its normal to hear a subtle hum or buzzing from the chassis pump, however, it may be noiser if the pump is worn or not mounted properly i.e. missing mounting grommets that insulate the noise and vibration. The tank is easy to drop on this era Seville. I did mine on the ground as well as replacing the pumps raised with a floor jack and jack stands.   

[35-709]

My opinion ---
#1. You REALLY need a new/different mechanic, one who doesn't mind doing a little research and is not lazy.
#2.  Based on your #2 question, you are a whole lot better at figuring things out than your mechanic.  BTW, a clogged fuel filter will also cause your frame mounted pump to be noisier.
#3.  Answered above
#4.  Certainly should be, but best answered by someone else.

Show your mechanic the pictures you posted here and see what he says.  HE should be able to figure out, or know, how to test the in tank pump.  Sounds like you are paying for his education and he isn't sure he wants to learn.   

[35-709]

Reason why many of us learned to be our own mechanic on these older cars, if we weren't already, as Scott Minesinger has pointed out here many times. 
Those interested in old cars but are non-mechanical types are doomed to paying for their "mechanics" education, mechanics who often are not really interested in old cars and don't have the time, need, or inclination to learn.  It can be an uphill, expensive and frustrating climb all the way when your mechanic rolls his eyes and says, "oh, no" to himself when he sees you coming.

[79 Eldorado]

J.C.
There's a Seville online documentation site which has a lot of good information. Just in case you haven't seen it here's the link:
http://www.cadillacseville.org/

One additional thing to check/change if the tank does come down is the small piece of submersible fuel line going from the in-tank pump to the fuel supply line exiting the tank. I'll mention it because I didn't realize it myself until about 5 years ago; that is submersible fuel line is a specific type. If you have trouble finding it I know Gates sells it in short lengths. I don't know the diameter needed but I was able to find some on Amazon.

You could probably determine if the in tank pump is working without dropping the tank but if the soft line is cracked the pump could be running fine; just not helping as much as it should. One other thing to remember when checking things out is the fuel pumps turn on with key-on but only for about 1 second. If you leave the key in the "on" position beyond 1 second, but the car is not running, the pumps will not continue to run.

Bruce Roe also alerted me to something recently which Cadillac overlooked and does cause a vibration. I would need him to explain it one more time for me to catch what component is vibrating/resonating but essentially something in the electronics which normally handles microsecond delays needs to deal with the 1 second fuel pump on and because of that something in the ECU can go into a resonance type vibration interacting somehow with the fuel pump relay. I think the relay itself is what ends-up vibrating. I guess you know the vibration is from the chassis pump but what you wrote made me wonder if you heard something and are now "tuned-in" to this other vibration. Maybe Bruce will see this and add a little more "meat" to my description. I thought it was stunning. He told me a friend in Poland who is also very knowledgeable with our cars alerted him to it.

Scott

[J.C.]

a clogged fuel filter will also cause your frame mounted pump to be noisier.

Can a clogged fuel filter be cleaned, or does it need to be replaced?

[35-709]

Can a clogged fuel filter be cleaned, or does it need to be replaced?

Replaced.

[J.C.]

It's normal to hear a subtle hum or buzzing from the chassis pump, however, it may be noisier if the pump is worn or not mounted properly i.e. missing mounting grommets that insulate the noise and vibration.

Do you mean that it's normal to hear it inside the car while driving, or do you mean it's normal to be able to hear it from outside the car at idle?  I ask because the sound is bugging me and I'm struggling a little with the idea that in Cadillac's top-of-the-line model that year I'd hear something like that. 

I had a '79 Coupe DeVille and a '79 Fleetwood, and I never heard any sounds like this in them, though, granted, they weren't fuel-injected.  My grandfather had a '76 Seville and I don't remember hearing a sound like this there, either (but, then, I was probably too young then to notice or care).

[79 Eldorado]

Replaced.

J.C.
To clarify this you would be replacing a paper filter element inside the housing; not an entire filter/canister/housing. I am assuming the Seville has the same style as my 79 Eldorado but if yes a canister unscrews and you replace the o-ring and the paper element. It's not difficult. The canister is almost the same size as the Oldsmobile engine oil filters.

Scott

[bcroe]

. Do you mean that it's normal to hear it inside the car while driving, or do you mean it's normal to be able to hear it from outside the car at idle? 
I had a '79 Coupe DeVille and a '79 Fleetwood, and I never heard any sounds like this in them, though, granted, they weren't fuel-injected.  My grandfather had a '76 Seville and I don't remember hearing a sound like this there, either 

There is a low pressure tank pump, probably borrowed from carb service. 
It is needed to keep gas coming, but the frame mounted pump boosts the
pressure to what the injectors need.  Pictured is what mine looked like several
years ago, that special submersible hose was completely sheared.  Best to
get the special hose and replace it, the frame pump may be pulling some air
or already has been damaged . 

There is no frame filter in a carb car, to make noise.  If the frame pump has
proper cushioned  mounts, the noise should not be too bad, maybe it is
already damaged.  Some of us solved the problem by mounting a high
pressure pump in the tank, with only an in line filter outside.  A bit of an
adventure in that environment. 

The fuel pump circuit in this ECU, as well as the Cosworth Vega of that time,
had design issues.  One was burn out, which can be avoided or fixed with
an external relay (used in all EFI 1980 on).  There is also a tendency of
the on/off to buzz some in operation, I fix this inside every ECU that comes
to me for any reason.  Not fatal but wears things out faster.  good luck,
Bruce (79 EFI owner for one quarter century) Roe CLC # 14630

[V63]

FYI!
I have had a number of these Bosch systems , including Mercedes. (And rolls Royce) I FINALLY learned that an over looked problem with these systems...specifically fuel pumps, is they do NOT like alcohol blended fuels!!! The alcohol causes the fuel pumps to operate at a higher mechanical operating resistance, and consequently electrical resistance ...Over loads and shuts down. Engine  stops, no restart.  45 minutes later the car will start like nothing is wrong. After countless fuel pump replacements with same results ...I found adding Marvel Mystery oil (or ATF, automatic transmission fluid) to the fuel SOLVES the problem (2oz per 5 gallon) ...it simply lubricates the fuel pumps and reduces BOTH mechanical and electrical resistance.

I had this epiphany after years of dealing with this problem. I bought a 10k mile pristine vehicle and driving it cross country and just 500 miles in it failed with a complete shut down, (no fuel pump operation heard) no restart. 45 minutes later....starts like normal and drives fine. Then an hour later stranded again. Towed to hotel and starts fine again 🤦🏻‍♂️.

I had the epiphany that night, did nothing more that supplement marvel mystery oil (or ATF) to the fuel and I NEVER had the issue again with that car some 15k miles when I sold it and advised owner to continue same, he drove it too without issue.

My Roll Royce manual with Bosch system advised simply (DO NOT USE ALCOHOL BLENDED FUELS!), so they were aware to the issue.

I also learned that the Submerged  fuel hose in tank is OFTEN replaced with regular fuel hose and it turns to goo, causing failure there.